The invention relates to a method for reducing the background count rate in radioactivity measurements which are evaluated using coincidence-anticoincidence techniques, in which pulses from a sample detector and pulses from a guard or shield counter are amplified and compared based on a time relationship in order to identify pulses of interest. The invention also relates to a circuit arrangement for performing the method.
In practice, problems exist when measuring very low activities of radiological substances, for example the hydrologically important classification of ground water by age, which in the range between approximately 100 and 1500 years can be done only by determining the Ar.sup.39 activity in the gas dissolved in the ground water. In recent waters (surface waters), there is an Ar.sup.39 content in the gas dissolved in the water of 0.11 dpm per liter of argon. For approximately 1000-year-old water, this drops in accordance with the half-life of (269 years) to approximately 0.008 dpm Ar.sup.39 per liter of argon.
For great constancy in the resulting count-rate, counting times of up to 10,000 minutes (that is, almost a week) for one sample are required, and the background radioactivity sensed by the measuring system must be very low. It will be apparent that these conditions place severe demands on the counter arrangement that is employed.
Efficient measurement of the Ar.sup.39 contents in the prepared argon can be done via a counter tube (e.g., a Geiger-Muller tube) as a sample detector. This counter tube is filled with prepared argon, with an admixture of CH.sub.4 as a quenching gas. The Ar.sup.39 activity of the argon in the counter tube is picked up as a pulse rate and compared with the count rate of argon that is free of Ar.sup.39 (zero sample). The information gained from initial zero sample measurements obtained during a measurement comparison has been reported in an article by Loosli, Forster and Otlet, "Background Measurements with Different Shielding and Anticoincidence Systems," Radiocarbon, volume 28, number 2A, Trontheim, (1986) pages 615-622. For operation of an Ar.sup.39 measuring arrangement in a near-surface laboratory, it is worthwhile not only to provide active shielding (as will be discussed) of the sample counter tube with a surrounding plastic scintillator, but also to effect a further reduction in the zero sample count rate. In initial measurements, it has been possible to accomplish this at present with an additional passive shield of lead surrounding the counter tube. This leads to the conclusion that part of the contributing zero effect can be ascribed to environmental gamma radiation not actively detected by the plastic scintillation counter.
This non-detected gamma radiation can derive from the residual activity of the materials used, but can also be generated in the material surrounding the counter tube, for example by cosmic muons.
As was discussed above the count attributable to background radiation can be reduced but not eliminated entirely by placing the counter tube which contains the Ar.sup.39 sample inside a passive shield, such as a lead enclosure. However the Ar.sup.39 sample is a feeble radioactive source, and further steps are desirable to ensure that the count attributable to decay of the Ar.sup.39 is not overwhelmed by the count attributable to the residual background radiation. For this purpose the well known coincidence-anticoincidence evaluation technique may be employed to provide what may be deemed an active shield inside the passive shield. To this end an additional radiation detector such as plastic scintillation counter is placed around the counter tube as a guard or shield counter. The arrangement is such that most background radiation rays which pass through the counter tube must also pass through the scintillation counter, so that a pulse from the counter tube can be attributed to the background radiation if it is accompanied by a pulse from the scintillation counter (that is, the pulses are coincident). On the other hand Ar.sup.39 decays by emitting a beta particle, which is detected by the counter tube but which is not sufficiently energetic to penetrate into the scintillation counter. Consequently a pulse from the counter tube which is not accompanied by a pulse from the scintillation counter (that is, the pulses are anticoincident) is attributable to decay of an Ar.sup.39 nucleus. Of course anticoincidence also arises if the scintillation counter emits a pulse and the counter tube does not, as when a background radiation ray traverses the scintillation counter and misses the counter tube, but this case can be ignored. In short the scintillation counter provides an active shield not in the sense of blocking residual background radiation within the passive shield, but in the sense of signalling moments of background activity so that pulses from the counter tube during these moments can be ignored.
The measuring method usually used can be explained in principle, for better comprehension of the problems involved, by referring to FIG. 1. FIG. 1 shows an anticoincidence measuring arrangement with conventional electronic components. In FIG. 1 characteristic output waveforms for various components are illustrated beside the respective component (or, where two waveforms are illustrated beside a component, both an intermediate waveform and the output waveform). The measuring system comprises a sample detector such as counter tube 1, into which the sample was pinched off, and a shield counter such as plastic scintillation counter 2 surrounding the counter tube 1, which is connected in anticoincidence with the counter tube 1. A passive shield (not illustrated) of lead surrounds counter tube 1 and scintillation counter 2.
Pulses from a photomultiplier 3 are temporally stretched and amplified by a preamplifier 4 and a main amplifier 5. If the pulse following the main amplifier 5 exceeds an adjustable voltage threshold, then a TTL standard signal of 5 volts in height and 500 nsec in length is emitted by a single-channel analyzer 6. Since with temporally coincident events the counter tube 1 responds more slowly than the scintillation counter 2, the signal from the scintillation counter must be correspondingly temporally delayed. This is done in the delay unit 7. Depending on the type of particle detected (ionization capacity, speed, and range), the signal from the scintillation counter is subjected to a time fluctuation in the .mu.s range. In order to attain a reliable voltage of a coincident counter tube pulse even with temporal migration of the signal from the scintillation counter, the standardized and then delayed TTL pulse of 50 .mu.s stretched to 150 .mu.s (stretch unit 8), before it is passed on to the coincidence-anticoincidence unit 9.
Analogously to the electronics of the plastic scintillation counter 2, a counter event detected by the counter tube 1 is also initially preamplified (10). Because of how the high counter tube voltage is typically connected, the signal must be inverted prior to the main amplification 11. The ensuing steps in the single-channel analyzer 12, in the delay unit 13, and in the stretch unit 14 take a similar course to that for the signal from the scintillation counter, except that for the counter tube 1 a stretch to approximately 40 to 50 .mu.s is performed, in order to suppress any possible afterpulses of one counting event. In the coincidence-anticoincidence unit 9, a comparison of the scintillation counter and counter tube event takes place in accordance with their time relationship. If the scintillation counter signal and the counter tube signal are simultaneous within 150 .mu.s, then the counter tube event is summed up as "coincident." Analogously, a counter tube event is summed up as "anticoincident" if it does not have any accompanying pulse from the scintillation counter. Via the scaling unit 15, a new measurement cycle is started, after which the results are output to a printer 16.